1. Field of the Invention
The present invention generally relates to an image formation method that uses electrophotography, iono-graphy, and magneto-graphy, and the like, and especially relates to the image formation method wherein a plurality of toners are laminated by developments, and then, all the laminated toners are fixed in one operation that employs radiant energy of a flashlight.
2. Description of the Related Art
Electrophotography is widely used in image formation apparatuses, such as copying machines, facsimile machines, and printers. Electrophotography often employs a method that uses an optically conductive insulator, as indicted by U.S. Pat. No. 2,297,691, etc. In this method, an electrostatic latent image is formed by irradiating light from a luminous source, such as a laser and a LED, to the optically conductive insulator that is electrified by a corona electric discharge, or by an electrically charging roller. Then, resin powder colored by pigments and dyes (hereinafter called colorant), which is called toner, is adhered to the latent image by an electrostatic force, and the latent image becomes a visible toner image. Then, the toner image is transferred to a recording medium, such as paper and film. However, the toner image at this stage is only an image of powder that is simply placed on the recording medium. The powder needs to be fixed on the recording medium. For this purpose, the toner is fused on the recording medium by heat, pressure, light, etc., and then the toner solidifies. In this manner, the toner image is finally fixed to the recording medium.
As described above, fixing of the toner usually refers to fusing the toner that is powder mainly consisting of thermoplastic resin (called binder resin hereinafter) by heat, and then, adhering the toner to a recording medium. Well-known fixing methods include a heat roll method, wherein a roller applies heat and pressure directly to the recording medium on which the toner image is formed. Another well-known method is a flashlight fixing method wherein the toner is fixed on the recording medium by irradiating flashing light of a lamp such as a xenon flash lamp.
The flashlight fixing method transforms radiant energy of sparkling light (flashlight) from a discharge tube, such as the xenon flash lamp, into thermal energy, which is used to fuse the toner. Then, the toner is fixed to the recording medium
The flashlight fixing method when used in the image formation apparatus has the following features, as compared with the heat roll method.
(1) Fixing is performed without contacting the toner image, therefore, resolution of the toner image formed on the optically conductive insulator is not degraded.
(2) The flashlight fixing does not require any warming up at starting, providing a quick start for an operation.
(3) The flashlight fixing is not excessively sensitive to material and thickness of the recording medium, whether it is paper with adhesive, pre-printed paper, variable thickness paper, and the like.
A process in which the toner is fixed on the recording medium by the flashlight fixing is as follows. The radiant energy of the flashing light emitted from the discharge tube is absorbed by the toner image (powder image) that is formed on the recording medium, and is converted into thermal energy. Thereby, the temperature of the toner rises, the toner is softened and fused, and is stuck to the recording medium. When the flashing light is turned off, the temperature falls, and the fused toner solidifies and is fixed to the recording medium, producing the fixed toner image.
However, a xenon flash lamp that is usually employed as the discharge tube for the flashlight fixing emits light over a wide range of wavelengths such as between 400 nm and 1400 nm. The luminous intensity of the xenon flash lamp is considerably higher in a range of 800 nm-1400 nm, which is in the near-infrared wavelength domain, compared with the luminous intensity in the visible wavelength range, i.e., 400 nm-800 nm. For this reason, the toner used in the flashlight fixing is required to have high radiant energy absorption properties in the wavelength range of 800 nm-1400 nm, that is, the near-infrared wavelength domain.
However, generally the binder resin that is one of two main ingredients of the toner has a remarkably low radiant energy absorption nature in the visible and the near-infrared wavelength domains. As for the second ingredient, that is, the colorant, black colorant shows a high radiant energy absorption property in both the visible and the near-infrared domains. However, colorants of different colors, such as yellow, cyan, magenta, red, blue, and green, show a high radiant energy absorption property in the visible wavelength domain, but have a low radiant energy absorption property in the near-infrared domain.
For this reason, color toner containing the binder resin and the colorant for colors requires a higher intensity of the flashing light to fix an image than required by the black toner.
To solve the problem as mentioned above, it has been proposed that an infrared light absorbent material that absorbs the radiant energy in the luminous wavelength range of the xenon flash lamp be added to the colorants in order to reduce the amount of radiant energy required for fixing the color toner on the recording medium by the flashing light. For example, in JP S61-132959A, JP H06-118694A, JP H07-191492A, and JP 2000-147824A, disclosures have been made about adding materials to the toner for the flashlight fixing, the materials being an aminium system compound, a dimonium system compound, and a naphthalo-cyanine system compound. Moreover, JP H06-348056A discloses a technology of adhering resin granules to the surface of the toner, the resin granules containing an infrared light absorbing material such as an anthraquinone system, a polymethine system, and a cyanine system. Furthermore, a technology of improving the fixing properties of the color toner in the flashlight method has been published by JP H10-39535A, in which tin oxide and indium oxide are added to the color toner.
The technologies indicated above attempt to improve efficiency in converting energy from radiant energy to thermal energy. For this purpose, the infrared light absorbent is added to the color toner such that the fusion property of the binder resin, which is the main ingredient, is enhanced.
However, the conventional technologies, which, in essence, simply add the infrared light absorbent, have not solved all the problems described above. Specifically, adding a small amount of the infrared light absorbent material does not sufficiently improve the fusion properties of the binder resin. However, the materials that are added to enhance infrared absorption, namely, the aminium system compound, the dimonium system compound, and the naphthalo-cyanine system compounds are colored by nature. That is, if a large amount of these materials is used, chroma and hue of a color image is adversely affected. Therefore, it is desired that the amount of the infrared light absorbent be as small as possible.
As mentioned above, the flashlight method of the conventional technology still requires a large amount of radiant energy in order to firmly fix the color toner.
The above problem of needing the large amount of radiant energy becomes a bigger problem in multi-color and full-color image formation, when a “sequential developments and one-time fixing” method is used, wherein a plurality of toner powder image layers are laminated, and then the laminated layers are fixed by one shot.
In the “sequential developments and one-time fixing” method, greater radiant energy is required in order to firmly fix the toner of the laminated layers compared to fixing a single toner layer. In addition, even if the radiant energy is simply increased for fixing the laminated layer, it is difficult to obtain firm toner fixing. It is because not only the total amount of the toner to be fused by irradiation of light is large, but also the flashing light is absorbed by upper toner layers, and lower layers that are closer to the recording medium, and therefore, more relevant to the fixing properties, do not receive sufficient energy for fusing.
For the above reason, energy from irradiation of more than two times is required of conventional color printers adopting the “sequential developments and one-time fixing” method, in comparison with radiant energy of monochrome printers of the same speed. In order to provide irradiation of such high energy, a large-scale irradiation unit is required.
Therefore, a problem is that an image formation apparatus tends to be large-sized, expensive, and the radiant energy absorption efficiency therein falls, being incapable of meeting recent requirements for higher speed in forming an image.
The present invention is made in view of the above problem. Therefore, the purpose of the present invention is to offer a “sequential developments and one-time fixing” method that provides a high-quality fixed image using less radiant energy.
The toner used in the electrophotography of the present invention can be properly used in other image formation methods such as iono-graphy and magneto-graphy.